One type of prior printer assembly is a scan printer assembly. The scan printer assembly includes a printhead and a paper advance system. The paper advance system generally includes a drive motor (often a stepping motor), a plurality of paper rollers and a mechanical transmission means that transmits the motion of the drive motor to the rollers. The rollers place and maintain a paper for printing on a paper platen of the printer assembly, facing the printhead. The printhead moves across the paper for printing in a direction perpendicular to the movement of the paper. The printhead prints one scan line at a time. The paper advance system advances the paper when the printhead finishes each scan line on the paper.
When printing, the printer assembly first sends control signals to the drive motor to rotate the motor for a specified time. The mechanical transmission means transmits the motion of the drive motor to rollers. The rollers rotate and drag the paper on the paper platen in a direction perpendicular to the movement of the printhead for a predetermined distance thereby forwarding the paper. The predetermined distance corresponds to the rotation of the drive motor during the specified time. Printhead control signals and data to be printed are then sent to the printhead. Under the control signals, the printhead moves across the paper and makes one scan printout on the paper. The paper is then forwarded by the paper advance system and the printhead makes another scan on the paper. The process is repeated until the printer assembly completes the printing on the paper.
One prior problem associated with this type of printer assembly is that the paper advance system of the printer assembly from time to time creates errors with respect to the movements of the paper for printing. The result is that the paper is mispositioned for printing, thereby causing printing errors.
In printing on paper, text and graphics can often be printed entirely on one scan line. However, large text (e.g. large fonts) and large graphics will often span several scan lines and thus several passes of the printhead over several scan lines will be required to print the large character or graphics. This means that during these several passes, that paper will be forwarded by the paper advance system after only portions of a character (or graphics) have been printed. It will be appreciated that paper advance errors (e.g.the rollers roll too far or too short) will cause printing errors because the top of one scan line will not meet the bottom of the next scan line. For example, if the rollers roll too far, there will often be a noticeable gap in the character being printed (the gap results from the unprinted, unwanted margin between the scan and the previous scan). If on the other hand, the errors of the paper advance system causes the paper movement for the scan to be short of the desired distance, a dark band appears where the prior scan and the current scan overlap. The errors are referred to herein as stitch errors. These stitch errors of the printer assembly degrade the printed text or graphic image and sometimes make them illegible.
One contribution in the printer assembly to the stitch errors is the runout of the paper rollers within the paper advance system. Roller runout is caused by the difference between the desired movement of a paper roller in the printer assembly and the actual movement of the roller. Roller runout is defined about a datum axis which is determined by the support shaft for the roller. Runout is the difference of any two radical points measured on the roller as it is rotated on the datum axis. One component of roller runout is caused by using elastomer rollers in the printer assembly. The elastomer rollers are employed in the printer assembly to produce high friction for the paper so that the paper tracks the movement of the rollers. Due to the elastic nature of the rollers, the radius of each of the rollers varies and runout occurs during the movement of the paper rollers. The runout can produce errors of several tenth of mils to several mils in paper movement.
One prior solution to this problem is to use non-elastomer materials for the paper rollers. However, one disadvantage of this solution is that a non-elastomer surface produces less friction and may leave marks on the paper passing through the paper rollers.
Another prior solution to the problem is to mold and grind the elastomer rollers to precise tolerances during the manufacturing process. It is, however, impractical to maintain precise tolerances with elastomer rollers because of the pliable characteristics of the elastic materials. The characteristics of the elastomer rollers may also change as they age. It is also noted that paper advance errors result from problems in the drive mechanism (e.g. runout of the motor capstan or runout of the motor shaft, etc.) and that errors from the drive mechanism are of the same order of magnitude as errors from roller runout. It will be appreciated that the total scan error is from the drive mechanism and roller runout.
Moreover, each individual printer assembly has a unique set of the errors. Also, the magnitude for each of the errors varies among individual printer assemblies.